The present invention relates to an exhaust gas clean-up catalyst, especially an exhaust gas clean-up catalyst capable of keeping a clean-up ability high.
In order to improve fuel consumption characteristic and exhaust gas characteristic, lean-burn engines such as lean-burn engines, in-cylinder fuel injection engines, etc. are operated at a lean air-fuel ratio which is leaner than the stoichiometric air-fuel ratio, in predetermined operating regions. While the engine is operated at a lean air-fuel ratio, NOx (nitrogen oxide) contained in exhaust gas cannot sufficiently be removed by a three-way catalyst. Thus, it is known to provide an NOx catalyst which occludes NOxcontained in exhaust gas in an oxidative atmosphere, and reduce the NOx occluded by the NOx catalyst to N2 (nitrogen) in a reducing atmosphere, to thereby reduce the emission of NOx into the atmosphere. As an occlusion-type lean NOx catalyst of this kind, there is one whose NOx occluding ability is improved by adding potassium (K), which is an alkaline metal, as an NOx occluding agent, as disclosed, for example, in Japanese Unexamined Patent Publication No. Hei 9-85093.
However, when the NOx catalyst having potassium added is placed in high temperature for a long time, cracks form in the catalyst in some cases. Such cracks lower the durability of the NOx catalyst.
In order to find out causes of the lowering of the durability of the NOx catalyst, the inventors of the present invention produced an NOx catalyst in which the above-mentioned potassium (K), which is an alkaline metal, was added as an NOx occluding agent to a catalytic layer supported by a honeycomb-type cordierite support (porous support), and carried out a bench test of an engine equipped with the NOx catalyst, and a driving test of a vehicle having such an engine installed. In the bench test and the practical vehicle driving test, the engine and the vehicle were operated and driven in the conditions in which the NOx catalyst was exposed to high temperature of 650xc2x0 C. and above for quite a long time. After the engine and the vehicle were thus operated and driven, elemental analysis of the NOx catalyst was carried out on its cutting plane by EPMA (Electron Probe Microanalysis). By this, it was found that in the cordierite (Mg2Al4Si5O18) layer of the catalyst existed KMg4Al9Si9O36, a compound of potassium, magnesium, aluminum, silicon and oxygen, and KAlSiO4, a compound of potassium, aluminum, silicon and oxygen.
From the above experiment, it is thought that when the NOx catalyst is exposed to high temperature, potassium which has been added to the catalytic layer (wash coat) penetrates into the cordierite support and reacts with cordierite in a high-temperature atmosphere to form the above-mentioned compounds. Here, it is thought that since potassium compounds are high in water solubility and low in melting point, potassium easily penetrates into the cordierite support. When compounds having a thermal expansion coefficient different from that of cordierite is formed in the cordierite support, cracks form in the cordierite support due to change in the catalyst temperature during, before and after the use of the catalyst, and thus the strength of the catalyst lowers.
As mentioned above, the NOx catalyst which contains potassium or the like as an occluding agent is used in an oxidative atmosphere. In the oxidative atmosphere, nitrate (xe2x80x94NO3) of the occluding agent is formed by chemical reaction of the occluding agent with nitrogen contained in exhaust gas, and hence, the NOx occluding ability lowers. Here, the occluding ability can be restored by forming a reducing atmosphere around the NOx catalyst and thereby degrading the nitrate. However, even when such measures are taken, the clean-up ability lowers in some cases in which the NOx catalyst is used in high temperature for a long time.
From the results of an experiment carried out by the inventors, it is thought that one of the causes of the clean-up ability lowering lies in the occluding agent gradually vaporizing and escaping from the NOx catalyst in high temperature, thus a large amount of the occluding agent in the catalyst getting lost. Specifically, the inventors produced an NOx catalyst in which a cordierite support supported a catalytic layer which contained potassium as an occluding agent, and obtained the amount of potassium contained in the unused NOx catalyst by XRF (X-ray Fluorescence Spectrochemical Analysis). Then, after using the catalyst in high temperature for a long time (for example, in 850xc2x0 C. for 32 hours), the amount of potassium contained in the catalyst was obtained. Then, the ratio of the lost potassium was obtained by dividing the difference between the amount of potassium contained in the unused catalyst and the amount of potassium contained in the after-use catalyst by the amount of potassium contained in the unused catalyst. It was found that the ratio of the lost potassium was several ten % to 50%.
In this connection, Japanese Unexamined Patent Publication No. 2000-279810 discloses a technique in which, for example, when alkaline metal such as potassium is used as an NOx occluding agent, silicon which has an affinity to alkaline metal is provided in a catalytic layer to restrain the movement of the alkaline metal into a base (cordierite support) and thereby retain the alkaline metal in the catalytic layer.
However, an occluding agent made of alkaline metal has, as its property, a large electron-donating function. Thus, there is a problem that by emitting electrons, alkaline metal lowers NOx oxidation performance by precious metal. Specifically, the NOx occluding catalyst makes NO react with oxygen and also with alkaline metal and occludes NOx in the form of the above-mentioned nitrate (xe2x80x94NO3), but the electron-donating function of alkaline metal hinders the reaction of NO with oxygen. Occluding agents which are higher in occluding ability such as potassium tend to be higher in electron-donating function.
Thus, when a larger amount of alkaline metal such as potassium is retained in the catalytic layer as in the technique disclosed in the above-mentioned publication, occlusion ability which depends on the reaction of NO with alkaline metal improves, while the oxidation reaction of NO does not go on well. Thus, balance between the reactions is lost, which leads to a problem that NOx removing ability lowers.
Another problem with the occluding agent made of alkaline metal such as potassium is that alkaline metal reacts with S (sulfur) contained in exhaust gas to form sulfate (xe2x80x94SO4), which deteriorates the NOx occluding ability. (This is called sulfur-poisoning.) Regarding this problem, it is known that sulfur in exhaust gas can be removed by making the catalyst high in temperature and making the exhaust gas rich in air-fuel ratio to thereby form a reducing atmosphere. (This is called sulfur-purging.) However, sulfur-purging has a problem that if the occluding agent moves and is retained deep below the catalyst surface with which exhaust gas flow comes in contact, the occluding agent which has turned into sulfate is not easily exposed to the high-temperature reducing atmosphere, and hence it takes time to remove sulfur.
The object of the present invention is to provide an exhaust gas clean-up catalyst in which movement of an occluding agent is restrained without lowering the oxidation ability, and degree of lowering of the exhaust-gas clean-up ability is much reduced, and which has high durability.
In order to achieve the above object, the present invention provides an exhaust gas clean-up catalyst comprising a support and a catalytic layer, said catalytic layer containing, as an occluding agent, at least one chosen from a group consisting of alkaline metals and alkaline earth metals, wherein said catalytic layer comprises a first catalytic layer which contains said occluding agent and an acid material having a high affinity to said occluding agent, and a second catalytic layer which contains said occluding agent and does not contain said acid material.
In this exhaust gas clean-up catalyst, since the occluding agent in the first and second catalytic layers combines with the acid material in the first catalytic layer, movement of the occluding agent into the support is restrained, and loss of the occluding agent due to vaporization and escape from the catalyst, etc. is prevented. Here, since the occluding agent in the second catalytic layer moves into the first catalytic layer to combine with the acid material and is retained in the first catalytic layer, the amount of the occluding agent contained in the second catalytic layer reduces. As a result, in the second catalytic layer, oxidation reaction by precious metal goes on well without being hindered by the occluding agent. The oxide formed by the oxidation reaction is occluded well by the occluding agent retained in the first catalytic layer. Thus, the lowering of the exhaust gas clean-up ability of the catalyst is prevented.
The NOx removing efficiency of the exhaust gas clean-up catalyst after long-time use in high temperature was examined actually. It was found that in the exhaust gas clean-up catalyst according to the present invention, the NOx removing efficiency was generally kept high, irrespective of the catalyst temperature, as compared with a case in which a simple catalytic layer having no acid material mixed was used (see FIG. 5).
It is to be noted that as long as the acid material can mix with the other materials in the first catalytic layer, the acid material may either be highly dispersed or exist in somewhat large particles or blocks. In any case, the lowering of the exhaust gas clean-up ability of the catalyst can be prevented satisfactorily.
In one mode of the exhaust gas clean-up catalyst according to the present invention, said second catalytic layer is formed on the outside of said first catalytic layer.
In this case, the occluding agent in the second catalytic layer moves into the lower first catalytic layer to combine with the acid material and is retained there, while in the second catalytic layer, oxidation reaction by precious metal goes on well without being hindered by the occluding agent. Thus, the lowering of the exhaust gas clean-up ability of the catalyst is prevented. Since the second catalytic layer is formed on the outside of the first catalytic layer, the second catalytic layer is exposed to exhaust gas flow, and thus the exhaust gas easily comes in contact with precious metal. By this, oxidation reaction by precious metal is promoted very well, so that the exhaust gas clean-up performance improves.
Meanwhile, the first catalytic layer which contains the acid material is not exposed to the exhaust gas flow. Thus, the acid material which is relatively low in thermal resistance is protected well by the upper second catalytic layer.
Further, since the occluding agent is retained in the first catalytic layer which is located deep below the catalyst surface, the loss of the occluding agent due to vaporization and escape from the catalyst, etc. can be prevented better.
In one mode of the exhaust gas clean-up catalyst according to the present invention, said second catalytic layer is formed between said support and said first catalytic layer.
In this case, the occluding agent in the second catalytic layer moves into the upper first catalytic layer to combine with the acid material and is retained there, while in the second catalytic layer, oxidation reaction by precious metal goes on well without being hindered by the occluding agent. Thus, the lowering of the exhaust gas clean-up ability of the catalyst is prevented. Since the first catalytic layer which contains the acid material is formed on the outside of the second catalytic layer, the occluding agent gathers in the first catalytic layer which is exposed to exhaust gas flow, and the exhaust gas easily comes in contact with the occluding agent. Thus, the occluding agent and sulfur contained in the exhaust gas react easily, and the catalyst suffers sulfur-poisoning easily. However, this also means that in sulfur-purging which is carried out by making the catalyst high in temperature and making the exhaust gas rich in air-fuel ratio to form a reducing atmosphere, the occluding agent combined with sulfur is easily exposed to the high-temperature reducing atmosphere. Thus, sulfur is purged easily, and the length of time in which the catalyst needs to be kept at high temperature and the exhaust gas needs to be kept at a rich air-fuel ratio can be shortened. As a result, deterioration of fuel economy can be prevented.
In one mode of the exhaust gas clean-up catalyst according to the present invention, said first catalytic layer further contains at least one precious metal chosen from a group consisting of platinum, rhodium and palladium, and alumina as a base material for said precious metal and said occluding material.
When the acid material becomes a base material for precious metal, the catalytic ability of precious metal tends to lower. However, when alumina is contained and functions as a base material for precious metal, the acid material becoming a base material for precious metal is restrained. As a result, in the first catalytic layer, the lowering of the catalytic ability of precious metal is prevented.
In one mode of the exhaust gas clean-up catalyst according to the present invention, the proportion of said acid material to said alumina is in the range of 1 to 30 wt %.
In this case, when precious metal, the occluding agent, the acid material, alumina, etc. are mixed into slurry for forming the first catalytic layer, the proportion of the acid material is smaller than the proportion of alumina. Thus, most of the precious metal, etc. are supported by alumina, and precious metal supported by the acid material is much reduced. Since alumina surely functions as a main base material, the lowering of the catalytic ability of precious metal in the first catalytic layer is prevented.
Further, since it is ensured that precious metal is supported well by alumina, precious metal, the occluding agent, the acid material, alumina, etc. may be mixed at a time to form slurry. Precious metal does not need to be supported by alumina in advance. Thus, the process of forming the catalytic layer can be simplified.
In one mode of the exhaust gas clean-up catalyst according to the present invention, said second catalytic layer further contains at least one precious metal chosen from a group consisting of platinum, rhodium and palladium, and alumina as a base material for said precious metal and said occluding material.
In this case, apart from the acid material, the materials for the second catalytic layer is the same as those for the first catalytic layer. Thus, if, for the second catalytic layer, the acid material is supported by the cordierite support in advance, the first and second catalytic layers can be formed using the same slurry. This can reduce the production cost.
In one mode of the exhaust gas clean-up catalyst according to the present invention, the first catalytic layer contains at least one material chosen from a group consisting of acid oxides which contain at least one acid substance chosen from among IV-, V-, and VI-group transition elements and IV-, V-, and VI-group typical elements, compound oxides which contain said at least one acid substance, materials which do not hinder reactivity of nitrogen oxide and said occluding agent, and materials which absorb reducing substances.
For example, the first catalytic layer is formed to contain at least one material chosen from a group consisting of acid oxides and compound oxides which contain at least one acid substance. Here, those acid oxides and compound oxides each contain at least one acid substance chosen from a group consisting of IV-, V-, and VI-group transition elements and IV-, V-, and VI-group typical elements.
In this case, the acid oxides and compound oxides which have each a high ability to fix the occluding agent and high thermostability can prevent the loss of the occluding agent and thereby prevent the lowering of the exhaust gas clean-up ability of the catalyst and improve its durability. Here, as a compound oxide, one which is a combination of oxides that has an acid center is desirable.
Here, at least one acid substance should be chosen considering its reactivity with the occluding agent. For example, when the occluding agent is potassium, it is desirable to use acid oxides and/or compound oxides which contain silica and/or tungsten as an acid substance.
As another example, the first catalytic layer is formed to contain an acid material which does not hinder reactivity of NOx and the occluding agent. In this case, movement of the occluding agent on the catalyst is restrained, and the occluding agent performs its NOx occluding function well.
As another example, the first catalytic layer is formed to contain an acid material which absorbs reducing substances (for example, reducing gas such as HC). In this case, by the reducing substances trapped due to the reducing-substance absorbing ability, nitrate and sulfate in the first catalytic layer are degraded, so that the NOx occluding ability is restored.
In one mode of the exhaust gas clean-up catalyst according to the present invention, the occluding agent contains potassium, and the support is a porous support.
When potassium is added as the occluding agent, movement of the occluding agent is restrained well by the acid material. Also, the loss of the occluding agent due to vaporization and escape is prevented, so that the lowering of the exhaust gas clean-up ability of the catalyst is prevented satisfactorily. Further, when the porous support is used, pressure loss of exhaust gas decreases, and exhaust gas comes in contact with the catalytic layer well. Thus, exhaust gas clean-up goes on well. It is to be noted that in the case of a catalyst comprising a porous support, generally, movement and vaporization and escape of the occluding agent happen easily, because exhaust gas containing high-temperature vapor flows through the catalyst better. However, such movement and vaporization and escape of the occluding agent are well prevented by the acid material.
Actually, regarding an engine equipped with an exhaust gas clean-up catalyst according to the present invention in which a cordierite support was used as a porous support and an acid material was mixed in the catalytic layer containing potassium as an occluding agent, a bench test and a vehicle driving test were carried out. After the tests, elemental analysis of the catalyst was carried out on its cutting plane by EPMA. It was found that the amount of potassium contained in the cordierite support was far smaller as compared with a catalyst having a simple catalytic layer in which an acid material was not mixed.
The amount of potassium contained in the catalytic layer after long-time use of the exhaust gas clean-up catalyst in high temperature was measured by XRF. It was found that quite a larger amount of potassium remained in the catalytic layer as compared with a catalyst having a simple catalytic layer in which an acid material was not mixed (see FIG. 3).
This result of experiment shows that potassium was well kept in the catalytic layer, and that escape of potassium and penetration of potassium into the cordierite support were restrained sufficiently.
The reason why a large amount of potassium remains in the catalytic layer is thought to be that the acid material is mixed in the catalytic layer. Specifically, it is thought that due to the affinity of the acid material, potassium is dispersed and attracted by particles of the acid material and thus retained well in the catalytic layer.
To sum up, when the occluding agent contains potassium, the occluding agent can be retained well in the catalytic layer without getting lost due to movement and escape. Thus, formation of compounds, which is caused by the occluding agent which has penetrated into the porous support, is restrained, so that formation of cracks in the porous support is prevented satisfactorily. As a result, the durability of the exhaust gas clean-up catalyst improves, and the exhaust gas clean-up ability is maintained satisfactorily.
In one mode of the exhaust gas clean-up catalyst according to the present invention, the first catalytic layer contains zeolite.
When the first catalytic layer contains zeolite which, as an acid material, has an affinity to the occluding agent and also has a cation exchange ability, additional advantages are obtained besides the above-mentioned ones.
Specifically, in some cases, in the presence of high-temperature vapor, the occluding agent which moves within the catalyst is ionized. When zeolite is contained in the first catalytic layer, the ionized occluding agent is fixed as ions due to the cation exchange ability of the acid center of zeolite (see FIG. 4), and thus hindered from moving toward the support.
Further, zeolite, which has a three-dimensional network structure and a large specific surface area, has a molecular sieving function, and the occluding agent is highly dispersed over this structure of zeolite. Thus, the occluding agent is less likely to penetrate into the support.
Even when the internal combustion engine is operating in a lean state, exhaust gas contains a small amount of HC. Meanwhile, zeolite has a high ability to fix the occluding agent and a high ability to absorb HC. HC absorbed by zeolite promotes degradation of nitrate and sulfate of the occluding agent. To sum up, even during lean operation of the engine, zeolite having an HC absorbing ability continuously degrades nitrate and sulfate of the occluding agent by using the small amount of HC which is contained in exhaust gas, and thereby contributes to restoration of the NOx occluding ability of the catalyst.